BISMARCK, N.D. — Donald J. Trump officially secured the number of delegates he needed to clinch the Republican presidential nomination Thursday, according to an Associated Press tally that showed unbound delegates putting him over the top.

Before a speech about energy in North Dakota on Thursday, Mr. Trump held a news conference intended to commemorate the symbolic elevation. He took questions from reporters for about 30 minutes, speaking with a backdrop of more than a dozen unpledged North Dakota delegates who are now supporting him.

“I’m so honored,” Mr. Trump said. “I’m so honored by these people. They had such great sense.”

Among the delegates behind Mr. Trump were John Trandem and State Representative Ben Koppelman, who were the final two to commit to Mr. Trump, helping to push him over the 1,237 delegate threshold. Each joked that he was the deciding delegate.

For Mr. Trump, whose remaining rivals dropped out the of the race this month, reaching the threshold puts an official capstone on a remarkable run in which he vanquished 16 other candidates, many of them with deep political experience. As recently as a month ago, the Republican race appeared to be heading for a contested convention, as a “Never Trump” effort sought to prevent him from winning enough support to get the nomination.

In his news conference, Mr. Trump — who in New Mexico on Tuesday night criticized Gov. Susana Martinez, a Republican, and called Mitt Romney “a choker” who walks “like a penguin” — also said he planned to unify the Republican Party.

“We have had tremendous support of almost everybody, and even if you look at Congress, the support has been incredible,” Mr. Trump said.

He added that he spoke over the phone Wednesday night with the House speaker, Paul D. Ryan, who so far has declined to endorse Mr. Trump. “We had a very good conversation that’s moving along,” Mr. Trump said. “He is a good man. We’ll see how that all works out.”

At an evening rally in Billings, Mont., Mr. Trump again celebrated his achievement: “Today was a big day,” he said, as the crowd cheered. “Today was the day we hit the 1,237.”

According to The A.P., Mr. Trump now has 1,238 delegates, putting him beyond the 1,237 needed for the nomination. There are more than 300 more delegates at stake on June 7 when California and four other states hold primaries.

Despite the fact that Mr. Trump won only a plurality of votes in many states, in part because of the size of the party’s field, the presumptive nominee regularly boasts that he has won a record number of votes for a Republican nominating contest.

While Mr. Trump has succeeded in winning a majority of the party’s delegates, some of his former rivals, including Senator Ted Cruz of Texas, Senator Marco Rubio of Florida, and Gov. John Kasich of Ohio, still control the delegates that they won during the nominating fight, potentially giving them influence over the direction of the party’s platform at the Republican convention July 18-21 in Cleveland.

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Hillary Clinton discussed her State Department email and criticized the presumptive Republican nominee, Donald J. Trump, during an interview in Las Vegas.CreditCreditSteve Marcus/Reuters

Ashley Parker reported from Bismarck, and Alan Rappeport from Washington.